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Cities plagued with terrible traffic problems may be overlooking a simple, low-cost solution: High-occupancy vehicle (HOV) policies that encourage carpooling can reduce traffic drastically, according to a new study co-authored by MIT economists.

Cities plagued with terrible traffic problems may be overlooking a simple, low-cost solution: High-occupancy vehicle (HOV) policies that encourage carpooling can reduce traffic drastically, according to a new study co-authored by MIT economists.

The results show that in Jakarta, Indonesia, travel delays became 46 percent worse during the morning rush hour and 87 percent worse during the evening rush hour, after an HOV policy requiring three or more passengers in a car was discontinued on important city center roads.

“Eliminating high-occupancy vehicle restrictions led to substantially worse traffic,” says Ben Olken, a professor of economics at MIT and co-author of a new paper detailing the study. “That’s not shocking, but the magnitudes are just enormous.”

Moreover, when the HOV policy was ended, traffic suddenly became significantly worse on surrounding roads as well. Instead of siphoning more traffic onto the central roads, the policy change made congestion worse all over.